This invention relates to a fluid dynamic pressure bearing adapted as a bearing for rotary apparatuses such as hard disc drive (HDD) units, a spindle motor used as a drive source for such rotary apparatuses and a fluid dynamic pressure bearing adapted for such spindle motors, and more particularly to a both-end fixed-shaft type fluid dynamic pressure bearing having a shaft to be fixed at its respective ends on a chassis, etc. of an apparatus utilized through screwing or the like.
Air dynamic pressure bearings are broadly used in rotary apparatuses such as an HDD, drive optical disc and light polarizing units because of their excellent merits such as their light weight, clean and smooth rotation, durability to heat and cold, long service life and noncontamination to a recording media such as a disc by virtue of not using lubrication oil. In recent years, however, there has been a significant increase in information to be processed. Particularly, are large capacity HDD apparatuses required to rotationally drive as many as five or more disc. This requirement can no longer be met by an air dynamic pressure bearing. In order to cope with this, fluid dynamic pressure bearings have been adopted in HDD apparatuses to support greater load weight than that supported by the air dynamic pressure bearings.
There are disclosures of the basic structure and operation of fluid dynamic pressure bearing , for example, in U.S. Pat. No. 5,112,142; U.S. Pat. No. 5,524,985; U.S. Pat. No. 5,524,986; and U.S. Pat. No. 5,533,812.
The conventional fluid dynamic pressure bearings, particularly fluid dynamic pressure bearings of the sleeve rotation, type include two kinds of devices depending on the ways used to fix the shaft onto an apparatus in which it is utilized. One type is a one-end fixed-shaft type fluid dynamic pressure bearing as shown in FIG. 13, and the other is a both-end fixed-shaft type fluid dynamic pressure bearing as shown in FIG. 14. First, the fluid dynamic pressure bearing of FIG. 13 is structured by a fixed shaft 1 at its lower end fixed on a chassis 16 or the like through a screw 15, and a rotary sleeve 2 having an upper end completely covered by a lid member 20 and a lower end having an opening 11 forming a capillary seal. Next, the fluid dynamic pressure bearing of FIG. 14 is structured by a fixed shaft 1 fixed at its opposite ends on a chassis 16 or the like of an apparatus utilized through screws 14 and 15, and a rotary sleeve 2 having openings 11a and 11b respectively forming upper and lower capillary seals.
In FIG. 13 and FIG. 14, 8, 8a, and 8b are radial dynamic pressure producing grooves while 9a and 9b are thrust dynamic pressure producing grooves. 5, 5a, 5b, 17a, 17b and 17c are fine gaps formed between the fixed shaft 1 and the rotary sleeve 2. These fine gaps are filled therein with lubrication oil 18. The fine gaps have a width of usually 2 to 15 xcexcm, although depending on the size of the fluid dynamic pressure bearing. 13a is an upper screw hole of the fixed shaft, while 13, 13b is a lower screw hole.
In the shaft-one-end fixed type fluid dynamic pressure bearing of FIG. 13, the lubrication oil 18 filled within the fine gaps 5, 17a, 17b and 17c contacts with the air at tapered opening 11. However, the lubrication oil 18 filled in the gaps is prevented from leaking outside the fine gaps by a capillary seal and surface tension due to the opening 11. In particular, the fine gaps 17a, 17b and 17c form a closed end.
The filled lubrication oil 18 hardly leaks out through the opening 11 due to a fine gap structure having such a closed end, i.e. a fine gap structure with one-side closure. In the both-end fixed-shaft type fluid dynamic pressure bearing of FIG. 19, on the other hand, the lubrication oil 18 filled within the fine gaps 5a, 5b, 17a, 17b and 17c contacts with the air at a tapered upper opening 11a and lower opening 11b. However, the filled lubrication oil 18 is prevented from leaking out of the fine gaps by the capillary seal and surface tension due to the openings.
Of the above related-art apparatus, the one-end fixed-shaft type fluid dynamic pressure bearing of FIG. 13 has a closed end in the fine gaps. Accordingly, the apparatus, in case tilted, hardly causes the lubrication oil to leak thus being excellent in sealability. However, there is a disadvantage in that the shaft 1 is fixed at only one point of its lower end and undergoes precession motion during rotation at high speed, resulting in instability in rotation. Conversely, the both-end fixed-shaft type dynamic pressure bearing of FIG. 14 fixes the shaft 1 at its both ends and hence does not undergo precession motion, offering stable rotation. However, there is a problem in that the fine gaps are opened to the air at upper and lower sides thus resulting in insufficient sealability. Even if a surface tension is formed by forming an air reservoir in a fine gap between the upper and lower radial dynamic pressure producing grooves 8a and 8b, the surface tension abruptly decreases when the fluid dynamic pressure bearing is tilted and positioned in a horizontal direction. Furthermore, when, in this state, temperature change or external impact is applied, the lubrication oil filled within the fine gap readily leaks to the outside.
It is an object of the present invention to maintain, in a both-end fixed-shaft type fluid dynamic bearing, a high sealability not only during rotation at high speed but also even upon being tilted in a standstill state.
It is another object of the invention to provide a spindle motor which can stably rotate at high speed.
It is still another object of the invention to provide a rotary apparatus which can stably rotationally drive at high speed a rotary member such as a hard disc.
In brief, the present invention is a double sleeve type fluid dynamic pressure bearing, comprising: a fixed shaft having respective ends to be fixed to an apparatus utilized; a rotary sleeve arranged to provide a first fine gap between an inner peripheral surface thereof and an outer peripheral surface of the fixed shaft; a fixed sleeve arranged to provide a second fine gap between an inner peripheral surface thereof and an outer peripheral surface of the rotary sleeve; and wherein the first fine gap and the second fine gap have one ends made as open ends contacting the air while the first fine gap and the second fine gap have the other ends made as closed ends in direct communication with each other, the fine gaps being filled with lubrication oil, and the first fine gap being formed with a dynamic pressure producing groove.
In the double sleeve type dynamic pressure bearing, the second fine gap is greater in width than the first fine gap within a range of capable of producing a dynamic pressure, thereby removing instability during high speed rotation due to a difference in flowing speed of lubrication oil.
In the double sleeve type fluid dynamic pressure bearing, seal means different from a related art capillary seal is provided at the opening of the second fine gap. The seal means is a resinous collar fitted at an outer end of the fixed sleeve. Alternatively, the seal means may be a curved type annular seal groove formed in the opening of the second fine gap by a first curved wall surface curved radially outward and a second curved wall surface similarly curved radially outward. Furthermore, the seal means may be a multi-staged slant type annular seal groove formed in the opening of the second fine gap by a first plurality slant wall surface having a plurality of annular slant surfaces slanted by stages radially outward and a second plurality slant wall surface having a plurality of annular slant surfaces similarly slanted by stages radially outward.
Also, the present invention is, in a spindle motor structured by a rotor including a rotor magnet, a stator including a stator coil and a fluid dynamic pressure bearing for rotatably supporting the rotor with respect to the stator, the spindle motor adopting for the fluid dynamic pressure bearing a double sleeve type fluid dynamic pressure bearing. The invention is furthermore a rotary apparatus having, as drive source to a rotary member, the spindle motor structured by a rotor including a rotor magnet, a stator including a stator coil and a fluid dynamic pressure bearing for rotatably supporting the rotor with respect to the stator.